Milakovitch Cycles and Climate Change
in the Recent Past

By the end of this
topic (reading and class discussion) you should be able to understand and
explain:

Why the Earth has seasons.

What the three Milankovitch Cycles are, as well as
their time periods and combined effect on insolation.

What the sunspot cycle is, and its effect on climate.

Introduction

Understanding
factors that affected past climates helps us gain perspective on present and
possible future climate change. This
reading introduces you to some factors that are highly predictable and that almost
certainly had an effect on Earth’s climate history.Detailed climate data do not exist for every
period of Earth’s history, so in this reading we will focus mainly on climate throughout
the last 500,000 years.This timeframe
is extremely useful because data for it are amazingly complete, and because many
of the same factors that affected Earth’s climate then also affect our climate
today.

Why The Earth Has Seasons

You would get
a surprising set of answers if you were to ask 100 people on the street to
explain in as much detail as possible what causes the seasons.Scientists pondered and studied this question
for many years before they finally worked it all out.The basic answer is that the Earth’s axis is
tilted in relation to the plane of the Earth’s orbit around to the sun, and
this produces seasons.If this is new
information to you, you may be thinking, “What does THAT mean?”

As you
know, the Earth spins on its axis.This
rotation gives us our cycle of night and day.Earth’s axis is, however, currently tilted 23.4o from
perpendicular in relation to the plane of its orbit around the sun (Fig.
1).This means that part of the year the
northern hemisphere is angled away from the sun and part of the year it is
angled toward it. During northern
hemisphere summer months the northern hemisphere is tilted toward the sun, and
therefore has longer days and receives a greater intensity of solar energy, technically
called insolation, than the southern
hemisphere (Fig. 2).During northern
hemisphere winter months the northern hemisphere tilts away from the sun and has
shorter days and receives a lower amount of insolation than the southern
hemisphere.This is what causes our
summers and winters.Of course, during
the spring (vernal) and fall (autumnal) equinoxes
the northern and southern hemispheres receive equal amounts of energy.

So why isn’t the hottest time of
year when we have the longest days, i.e., right around the summer
solstice?The oceans absorb a huge
amount of heat before the temperature of surface waters change very much.This characteristic of water causes the
actual timing of the hottest and coldest days, as well as the onset of spring
warming and fall cooling to be delayed slightly when compared to the dates of
equinoxes and solstices.

Figure 1.
Seasonal differences in the angle of the axis of rotation of Earth in relation
to the sun.(Image courtesy of DOE.gov)

Figure 2. The relative amounts of solar energy that
strike different latitudes of the Earth’s surface in June and December. (Image
modified from Dr. Gerken, NMSU.)

All right, but why is it always
warm in the tropics? Take another look
at Fig. 2 and you will see that the surface of the earth near the equator receives
a fairly constant amount of solar energy during all seasons of the year.This is why it’s warm there year-round.

Back to the
question about the reason we have seasons.Many people think that we have seasons because the Earth’s orbit is
elliptical, and that when Earth is closest to that Sun that’s when we have northern
hemisphere summer.While it’s true that
the Earth’s orbit is slightly elliptical, Earth is actually closest to the sun
in January!This means that the angle of
the Earth’s axis is more important to determining the seasons than the shape of
Earth’s orbit.What would happen if the
angle of Earth’s axis changed?Read on!

Milankovitch Cycles

The current
orientation of the Earth to the sun, and the shape of Earth’s orbit around the
sun do not tell the whole story of seasons and climate.It turns out that the orientation of the
Earth in relation to the sun and the shape of Earth’s orbit are not constant;
they undergo predictable cycles.As
these cycles occur they affect Earth’s climate.These factors are:

Precession – cyclic shifts in the
direction the Earth’s axis points.

Obliquity – cyclic shifts in the
angle of Earth’s axis.

Eccentricity - cyclic shifts in the
shape of Earth’s orbit around the sun.

These three constantly
shifting cycles and the way they interact with each other to affect Earth’s
climate are called Milankovitch Cycles.These cycles are named after Milutin
Milankovic.He did not discover these
cycles, but he studied the cycles and calculated how their interplay would impact
the relative position and orientation of the Earth to the sun, and how they
might affect climate.

FYI,
Milankovic was born in Serbia in the late 1800s, and he studied engineering and
science.His early work on this topic
was carried out while he was a POW during WWI, and the culmination of his
calculations and the theory they supported was published in a book in 1941 on
the eve of Nazi Germany’s invasion of Yugoslavia.His book was reportedly being printed when the
printing company was bombed, and only one copy of the original first edition
survived – the one he had taken for himself after the first day of the
publication run.

An introduction to the Milankovitch
Cycles is presented below.

Precession

A shift in
the direction the Earth’s axis points is called precession.Over time the direction pointed toward by the
Earth’s axis moves in the shape of a circle (see Fig. 3).This type of motion is much like the wobble
you see when a toy top starts to slow down. It takes about 23,000 years for the
Earth to undergo one complete cycle of precession.The Earth’s axis currently points toward
Polaris, also known as the North Star.That is the brightest star in the constellation Ursa minor (the little dipper).As earth’s precession continues the direction of the axis will shift
away from Polaris.Be advised that
precession alone does not change the tilt of Earth’s axis, only
the direction it points.

Figure 3. One cycle of Earth’s precession is
completed every 23,000 years.(Image: NASA.gov.)

Obliquity

The amount
of tilt in the Earth’s axis also shifts in a predictable cycle. The obliquity of the Earth’s tilt ranges from
22.1o to 24.5o (see Fig. 4), and is currently at an angle
of 23.45o in relation to the plane of its orbit around the sun.It takes about 41,000 years for Earth to
complete one cycle of obliquity. While the cycles of precession and obliquity
are taking place the shape of the Earth’s orbit around the sun also shifts.

Figure 4. The
range of obliquity of the Earth’s axis – current obliquity is 23.4o.
(Image: NASA.gov)

Eccentricity

The Earth’s orbit
around the sun is weakly elliptical, and the shape of this elliptical orbit also
undergoes a predictable cyclic change (see Fig. 5).Because the Earth has a slightly elliptical
orbit, the distance from the Earth to the sun is not the same all year
long.In fact, Earth is currently closest
to the sun on 3 January and is farthest away on 3 July.Think about that and the timing of seasons in
the northern hemisphere.

The
eccentricity of an orbit is perfectly circular when the orbiting object is
always the same distance from the object it is orbiting.A perfectly circular orbit has an
eccentricity value of 0.0.When an orbit
is elliptical, however, its eccentricity value is greater than 0.0.The range of Earth’s orbital eccentricity is
small, from a minimum of only 0.005 to a maximum of 0.058.Earth’s current eccentricity is about
0.02.In its current shape, Earth is 3%
farther away than when it is closest.It
takes about 100,000 years for Earth’s orbit to go through a complete cycle of
its eccentricity.

Figure 5 (right). This
figure shows the two extremes in the eccentricity of the orbit of one object
around another object; at one extreme its orbit is closer to circular (yellow
orbit), and at the other extreme it is more elliptical (red orbit). (Image: NOAA.gov)

Effects of Milankovitch Cycles

The
Milankovitch cycles are all happening at the same time, but they do not affect
the total amount of solar energy received by the Earth in a year.Together they do, however, combine to have an
effect on the intensity of solar radiation received at different
latitudes.For example, altering axis
tilt (obliquity) changes the intensity of seasons.

Look back at Figure 2 and take some
time and ponder on whether you think Earth would have more intense seasons when
its axis is 22.1o or 24.5o.

A change precession affects the
seasons in which Earth is closest to and farthest from the sun.Currently we are farthest from the sun during
the northern hemisphere summer. How might
that affect temperatures during high latitude winters?Summers?And, of course, shifts in the eccentricity of Earth’s orbit will also
have an effect on climate.

Climate and Milankovitch Cycles

The
importance of these cycles to climate seems to be in variations of intensity of
solar radiation received at higher latitudes at different times of the year,
since total solar radiation in the lower latitudes, i.e., near the equator,
remains constant.In addition, there is
greater total landmass in the northern hemisphere than in the southern, so the
climate effects will be largest in the north.This is because it takes more energy to raise the temperature of water
than it takes to change the temperature of land.A continent will therefore heat up and cool down
more quickly than an ocean.

Milankovic’ calculated the changes
in intensity of radiation received in high latitudes over long periods of time.He saw predicted short-term variability and
long-term changes in climate that corresponded to the three overlapping cycles
discussed earlier in this reading.He
hypothesized that shifts in the three Milankovitch Cycles might explain the timing
and extent of glacial and interglacial periods of the past few million
years.

Data from
polar ice cores allows scientists to collect observations needed to test Milankovic’s
hypothesis.In Figure 6 the yellow line
shows the calculated variations in solar insolation at 65o N over
the past 500,000 years based on Milankovic’s calculations.The lower graph in Fig. 6 indicates changes
in global climate obtained from ice cores.Glacial periods are in blue and interglacial or warm periods are in red.Notice that there is a correlation between
glacial and interglacial periods and changes in solar insolation due to
Milankovitch cycles.The connection is
statistically too strong to be ignored.These data support the explanation that there is a relationship between
Milankovitch Cycles and the cycles of ice ages and interglacial warm periods.If, however, climate were driven solely by
Milankovitch cycles we should currently be experiencing a trend of global
cooling.

Figure 6. Types
of Milankovitch cycles (top), calculated amount of solar insolation at 65oN
during the month of July (middle), and observed climate record from ice cores
based on oxygen isotope concentrations (bottom) with glacial periods indicated
in blue and interglacial periods in red.

Before we
assume that the primary cause and effect of recent ice ages and warm
interglacial periods is now sorted out, there are still some nagging
questions.First, the changes in insolation
intensity due to Milankovitch Cycles are quite small, and alone are probably
not large enough to cause such dramatic climate effects on their own.In addition, while eccentricity appears to
have the greatest influence on climate, it actually has the smallest impact on
insolation of the three cycles.So while
there is clearly a statistical correlation between the cycles and climate,
there are issues that have yet to be resolved.Do not, however, make that mistake of assuming that because science has
not yet answered all questions related to this complex question that we know
nothing about paleoclimate and driving forces of ancient and modern climate.The current interpretation is that small
changes in solar insolation may trigger planetary feedbacks that drive the Earth’s
climate system past tipping points that drive the planet into or out of ice
ages.

We will
return to ice core data, how these data are collected, what they represent, and
how they are interpreted later in the course.The reason they will not the handled in this reading is that there are
other foundational topics that you need to be introduced to before we discuss
ice core data.These include the
formation and effects of global surface winds, surface and deep oceanic
currents, and a more complete introduction to paleoclimatology and proxy
temperature data.But, before we leave
the topic of astronomical cycles that can affect climate there is one more
cycle that you may have heard of and that you will want to be aware of: the
sunspot cycle.

Solar Cycle: Sunspots

Scientists
discovered that the Sun’s output of solar energy fluctuates.This output varies naturally in conjunction with
the number of sunspots present at
any given time (see Fig. 7).Sunspots
were reportedly first observed by the naked eye in China as early as 28 BCE
(Before Common Era = BC), and we have a reliable scientific record of sunspots that
goes back to 1610.Scientists have been
using satellites to record the number of sunspots and solar output since
1978.These satellite data reveal that solar
output varies by about 0.08%, with the lowest output occurring when no sunspots
are present and the highest when many are present.

Sunspots are
regions on the surface of the sun that are cooler than the surrounding
surface.This is counterintuitive
because total solar irradiance increases when sunspots are present.Total intensity increases because whenever
sunspots form so do solar faculae.Solar faculae are bright, extra hot regions
of the sun’s surface where solar irradiance is higher than normal.

The 400-year record reveals that there
is an 11-year sunspot cycle.It also
shows that the maximum number of sunspots produced varies from cycle to cycle (see
Fig. 8).There were periods of time in the
400-year record when few to no sunspots were visible for extended periods of
time.It is possible that these
occurrences may represent a longer-term solar cycle, but data confirming or
refuting that possibility are still lacking.Extended periods of time when very few sunspots were present correlate
with periods of cooler temperatures on Earth.The longest recorded period of time when the sun produced no sunspots is
called the Maunder Minimum.It began in the mid-1600s and continued until
about 1700.During that time the Earth
experienced a cool period sometimes referred to as “The Little Ice Age.”

Figure 7. The sun
with sunspots in 2000 (left) and no sunspots in 2009 (right). (Image courtesy
of NASA.)

Figure 8. 400-year
record of sunspots. The Maunder Minimum
correlates with a cooling event called the “Little Ice Age”. (Image courtesy of
NASA.)

Most of the time sunspots appear
and disappear on the predictable 11-year cycle, but climatologists discovered
that total insolation differences between low and high points in a given cycle
account for only a small fraction of observed changes in global climate.The current sunspot cycle is shown in Fig. 9.These data show that we ended one sunspot
cycle in 2009-2010 and that we are now entering a new sunspot cycle.If our climate were driven solely by changes
in solar irradiance, 2000-2010 should have been a decade of global cooling, but
we observed the opposite.

Figure 9. The
number of sunspots present per month from January 2000 through September
2011.(Image courtesy of NOAA.org.)

Conclusion

Natural long-cycle events are now
well known, and climatologists continue to investigate how these factors
together with shorter-term events and other forcing factors affect Earth’s climate.